Method for the selective production of acetic acid by catalytic oxidation of ethane and/or ethylene

ABSTRACT

The invention relates to a method for the selective production of acetic acid from a gas-phase feed of ethane, ethylene, or mixtures thereof and oxygen at elevated temperatures. The gas-phase feed is brought into contact with a catalyst, containing the elements Mo, Pd, X and Y in the gram atom ratios a:b:c in combination with oxygen according to formula (I): Mo a Pd b X c Y d . The symbols X and Y have the following meanings: X=one or several elements chosen from the group Cr, Mn, Nb, Ta, Ti, V, Te and W; Y=one or several elements chosen from the group B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Ti and U; the indices a, b, c, d and x=the gram atom ratio for the corresponding elements, where: a=1; b=0.0001 to 0.01; c=0.4 to 1; and d=0.005 to 1. The space-time yield for the above oxidation to yield acetic acid is 470 kg/(hm 3 ). The selectivity of the oxidative reaction of ethane and/or ethylene to give acetic acid is, in particular, ≧70 mol %. X=preferably Nb and an ammonium salt of niobium is used as niobium source.

[0001] The present invention relates to a process for the selectivepreparation of acetic acid by catalytic gas-phase oxidation of ethaneand/or ethylene-in the presence of a molybdenum- andpalladium-containing catalyst.

[0002] The oxidative dehydrogenation of ethane to ethylene in the gasphase at temperatures >500° C. is disclosed, for example, in U.S. Pat.Nos. 4,250,346, 4,524,236 and 4,568,790.

[0003] Thus, U.S. Pat. No. 4,250,346 describes the use of a catalystcomposition comprising the elements molybdenum, X and Y in the ratioa:b:c for conversion of ethane into ethylene, where X is Cr, Mn, Nb, Ta,Ti, V and/or W, and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si,Sn, Tl and/or U, and a is 1, b is from 0.05 to 1, and c is from 0 to 2.The total value of c for Co, Ni and/or Fe must be less than 0.5.

[0004] The reaction is preferably carried out in the presence of addedwater. The catalysts disclosed can likewise be used for the oxidation ofethane to acetic acid, the efficiency of the conversion to acetic acidbeing about 18%, with an ethane conversion of 7.5%.

[0005] The abovementioned specifications are principally concerned withthe preparation of ethylene, less with the specific preparation ofacetic acid.

[0006] By contrast, EP-B-0 294 845 describes a process for the selectivepreparation of acetic acid from ethane, ethylene or mixtures thereofwith oxygen in the presence of a catalyst mixture which comprises atleast

[0007] A.) a calcined catalyst of the formula Mo_(x)V_(y) orMo_(x)V_(y)Z_(y), in which Z can be one or more of the metals Li, Na,Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Sc, Y, La, Ce, Al, Tl, Ti, Zr, Hf, Pb,Nb, Ta, As, Sb, Bi, Cr, W, U, Te, Fe, Co and Ni, and x is from 0.5 to0.9, y is from 0.1 to 0.4, and z is from 0.001 to 1, and

[0008] B.) an ethylene hydration catalyst and/or ethylene oxidationcatalyst. The second catalyst component B is, in particular, a molecularsieve catalyst or a palladium-containing oxidation catalyst.

[0009] On use of the catalyst mixture described and feeding of a gasmixture consisting of ethane, oxygen, nitrogen and steam through thecatalyst-containing reactor, the maximum selectivity is 27% with anethane conversion of 7%. The high conversion rates of ethane are,according to EP 0 294 845, only achieved with the catalyst mixturedescribed, but not in a single catalyst comprising components A and B.

[0010] A further process for the preparation of a product comprisingethylene and/or acetic acid is described in EP-B-0 407 091. In thisprocess, ethane and/or ethylene and a gas containing molecular oxygen isbrought into contact at elevated temperature with a catalyst compositioncomprising the elements A, X and Y. A here is Mo_(d)Re_(e)W_(f), X isCr, Mn, Nb, Ta, Ti, V and/or W, and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni,P, Pb, Sb, Si, Sn, Tl and/or U. The maximum selectivities which wereachieved on use of the catalyst described in the oxidation of ethane toacetic acid are 78%. Carbon dioxide, carbon monoxide and ethylene areformed as further by-products.

[0011] DE 19620542 describes a process for the selective preparation ofacetic acid from a gaseous feed of ethane, ethylene or mixtures thereofand oxygen at elevated temperature, wherein the gaseous feed is combinedwith a catalyst which comprises the elements a:b:c:d:e in combinationwith oxygen: Mo_(a)Pd_(b)Re_(c)X_(d)Y_(e), where the symbols X and Y aredefined as follows: X═Cr, Mn, Nb, B, Ta, Ti, V and/or W; Y═Bi, Ce, Co,Cu, Te, Fe, Li, K, Na, Rb, Be, Mg, Ca, Sr, Ba, Ni, P, Pb, Sb, Si, Sn,Tl, and/or U; the indices a, b, c, d and e are the gram-atom ratios ofthe corresponding elements, where a=1, b>0, c>0, d=0.05 to 2, e=0 to 3.In the examples given, ethane conversions of up to 8% and acetic acidselectivities of up to 91% are achieved at 280° C. and 15 bar.

[0012] DE 19630832 describes a process for the selective preparation ofacetic acid from a gaseous feed of ethane, ethylene or mixtures thereofand oxygen at elevated temperature. The feed is combined here with acatalyst which comprises the elements Mo, Pd, X and Y in combinationwith oxygen.

[0013] X here is one or more elements selected from the group consistingof Cr, Mn, Nb, Ta, Ti, V, Te and W, and Y is one or more elementsselected from the group consisting of B, Al, Ga, In Pt, Zn, Cd, Bi, Ce,Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf,Ni, P, Pb, Sb, Si, Sn, Tl and U. The gram-atom ratios for thecorresponding elements are indicated here as follows: a (Mo)=1; b(Pd)>0; c(X)>0; and d(Y)=0-2.

[0014] The catalysts described in the abovementioned application exhibita maximum space-time yield of 149 kg/(hm³) with an acetic-acidselectivity of >60 mol %. Space-time yields characterize the amount ofacetic acid produced per time and catalyst volume.

[0015] The invention WO 9847850 relates to a process for the selectivepreparation of acetic acid from a gaseous feed of ethane, ethylene ormixtures thereof and oxygen at elevated temperature on a catalyst whichcomprises the elements W, X, Y and Z in the gram-atom ratios a:b:c:d incombination with oxygen: W_(a)X_(b)Y_(c)Z_(d), in which X is one or moreelements selected from the group consisting of Pd, Pt, Ag and/or Au, Yis one or more elements selected from the group consisting of V, Nb, Cr,Mn, Fe, Sn, Sb, Cu, Zn, U, Ni and/or Bi, Z is one or more elementsselected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr,Ba, Sc, Y, La, Ti, Zr, Hf, Ru, Os, Co, Rh, Ir, B, Al, Ga, In, Tl, Si,Ge, Pb, P, As and/or Te, a=1, b>0, c>0, d=0 to 2, and to the catalystitself.

[0016] At 250° C., 15 bar and a residence time of 20 s, an acetic-acidselectivity of 80% was achieved with an ethane conversion of 10%.

[0017] WO 00/14047 relates to a process for the preparation of aceticacid in which ethane and/or ethylene is reacted with a gas containingmolecular oxygen in a fluidized-bed reactor in the presence of amicrospheroidally fluidized solid oxidation catalyst, where at least 90%of said catalyst particles are smaller than 300 μm. The catalystdescribed is a compound of the compositionMo_(a)W_(b)Ag_(c)Ir_(d)XeY_(f), where X is Nb or V, Y is one of theelements from the group consisting of Cr, Mn, Ta, Ti, B, Al, Ga, In, Pt,Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca,Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re and Pd; a to f are thegram ratios of the elements, where 0<a≦1, 0≦b<1, and a+b=1, 0<(c+d)≦0.1,0<e≦2, and 0≦f≦2. The examples given achieve a maximum space-time yieldof acetic acid of 354.4 kg/(m³h) for the oxidation of ethane to aceticacid and a space-time yield of acetic acid of 258.52 kg/(m³h) for theoxidation of ethylene to acetic acid.

[0018] In DE 19745902, it has been found that it is possible to oxidizeethane and/or ethylene to acetic acid under relatively mild conditions,in a simple manner and with high selectivity and excellent space-timeyields using a catalyst which comprises the elements molybdenum andpalladium and one or more elements from the group consisting ofchromium, manganese, niobium, tantalum, titanium, vanadium, telluriumand/or tungsten.

[0019] This invention DE 19745902 thus relates to a process for theselective preparation of acetic acid from a gaseous feed of ethane,ethylene or mixtures thereof and oxygen at elevated temperature, inwhich the gaseous feed is combined with a catalyst which comprises theelements Mo, Pd, X and Y in the gram-atom ratios a:b:c:d in combinationwith oxygen: Mo_(a)Pd_(b)X_(c)Y_(d); and the symbols X and Y have thefollowing meanings: X is one or more elements selected from the groupconsisting of Cr, Mn, Ta, Ti, V, Te and W, in particular V and W; Y isone or more elements selected from the group consisting of B, Al, Ga,In, Pt, Zn, Cd, Bi, Ce, Co, Cu, Rh, Ir, Au, Ag, Fe, Ru, Os, K, Rb, Cs,Mg, Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl and U, inparticular Nb, Ca, Sb and Li. The indices a, b, c and d are thegram-atom ratios of the corresponding elements, where a=1, b=0.0001 to0.01, c=0.4 to 1, and d=0.005 to 1. If X and Y are a plurality ofdifferent elements, the indices c and d may likewise adopt a pluralityof different values.

[0020] Said invention furthermore relates to a catalyst for theselective preparation of acetic acid, comprising the elements Mo, Pd, Xand Y in the gram-atom ratios a:b:c:d in combination with oxygen.Gram-atom ratios a:b:c:d are preferably in the following ranges: a=1;b=0.0001 to 0.005; c=0.5 to 0.8, and d=0.01 to 0.3.

[0021] Palladium contents in the catalyst which are above the statedupper limit have an advantageous effect on the formation of carbondioxide in the process described in DE 19745902. Furthermore, higherpalladium contents are generally also avoided because they unnecessarilyincrease the cost of the catalyst. By contrast, a preferential action onthe formation of ethylene is observed at palladium contents below thestated limit.

[0022] The catalyst used in DE 19745902 preferably comprises, besidesthe elements molybdenum and palladium, also vanadium, niobium, antimonyand calcium in combination with oxygen. The gram-atom ratiosa:b:c¹:d¹:d²:d³ for the elements Mo:Pd:V:Nb:Sb:Ca are preferably asfollows: a (Mo)=1; b (Pd)=0.0001 to 0.005, in particular 0.0001 to0.001; c¹ (V)=0.4 to 1.0; d¹ (Nb)=0.01 to 0.2; d² (Sb)=0.01 to 0.3;d³(Ca)=0.01 to 0.3.

[0023] The space-time yield achieved in DE 19745902 under Example 7 was470 kg/(hm³) at 310° C., 15 bar and a residence time of 7 s.

[0024] WO 00/00284 describes a catalyst system based on MoVNbPd, MoVLaPdor mixtures thereof for the preparation of acetic acid from ethylene.The examples indicated have a maximum space-time yield of 1291 kg/(m³h)with an ethylene conversion of 63.43% and an acetic-acid selectivity of78.03%.

[0025] The abovementioned patents make it clear that although space-timeyields of acetic acid of up to 1291 kg/(m³h) can be achieved in theoxidation of ethylene, the space-time yields of acetic acid in theoxidative reaction of ethane remain, however, significantly below thisspace-time yield which can be achieved for the oxidation of ethyleneowing to the greater difficulty in activating ethane compared withethylene. In DE-A-197 45 902, the greatest space-time yield of aceticacid achieved hitherto of 470 kg/(m³h) is achieved for the oxidation ofethane. Higher space-time yields are desirable, since this would allowthe size of the reactors and the amount of circulated gas to be reduced.

[0026] The object is therefore to provide a catalyst and a process whichallow ethane and/or ethylene to be oxidized specifically to acetic acidin a simple manner and with high selectivity and space-time yield underthe mildest possible reaction conditions.

[0027] The present invention describes a modified method for thepreparation of catalysts described in DE-A-197 45 902 of similarcomposition which result in improved catalytic properties of the statedcatalysts. One of the particular features of the present invention isthat, using the catalyst described, besides the oxidation of ethylenealso the considerably more difficult oxidation of ethane under optimizedreaction conditions results in high ethane conversions, high acetic-acidselectivities and particularly in high acetic-acid space-time yieldscompared with said patents.

[0028] In DE-A-197 45 902, the catalysts were prepared by a conventionalprocess starting from a slurry, in particular an aqueous solutioncomprising the individual starting components of the elements inaccordance with their proportions. The starting materials for theindividual components for the preparation of the catalyst according tothe invention were, besides the oxides, preferably water-solublesubstances, such as ammonium salts, nitrates, sulfates, halides,hydroxides and salts of organic acids which can be converted into thecorresponding oxides by warming. For mixing of the components, aqueoussolutions or suspensions of the metal salts were prepared and mixed. Inthe case of molybdenum, it was recommended, owing to the commercialavailability, to employ the corresponding molybdates, such as, forexample, ammonium molybdate, as starting compounds. The palladiumcompounds employed were, for example, palladium(II) chloride,palladium(II) sulfate, palladium(II) tetraamine nitrate, palladium(II)nitrate and palladium(II) acetylacetonate.

[0029] The present invention describes the preparation of the catalystby a different method using different starting materials than thosedescribed in DE-A-197 45 902. Thus, instead of niobium oxalate, use ismade of a niobium ammonium salt, preferably niobium ammonium oxalatehaving the composition X₃NbO(C₂O₄)₃+X₂NbO(OH)(C₂O₄)₂, where X═H⁺ or NH₄⁺ (manufacturer: H. C. Starck), which can have an Nb content of at least19% by weight, ammonia contents of from 0 to 12% by weight and typicaloxalate contents of from 50 to 65% by weight. Surprisingly, it has beenfound that the use of niobium ammonium salt, such as niobium ammoniumoxalate, as niobium source results in better catalytic properties, whichcan be essentially attributed to a different Nb distribution in thecatalyst compared with DE-A-197 45 902. In addition, the catalystpreparation method described in DE-A-197 45 902 is also modified in thatpalladium acetate is dissolved in an alcohol, in particular in ethanol,and not in acetone. This procedure also results in improved catalyticactivities, which is essentially attributable to improved distributionof Pd in the mixture as a whole.

[0030] The present invention thus also relates to a niobium-containingcatalyst of the abovementioned type which is obtainable by using aniobium ammonium salt, for example a niobium ammonium carboxylate, asniobium source.

[0031] The resultant reaction mixture is then stirred at from 50 to 100°C. for from 5 minutes to 5 hours. The water is subsequently removed, andthe catalyst which remains is dried at a temperature of from 50 to 150°C., in particular from 80 to 120° C.

[0032] In the case where the resultant catalyst is subsequentlysubjected to a calcination process, it is advisable to calcine the driedand powdered catalyst at a temperature in the range from 100° C. to 800°C., in particular from 200 to 500° C., in the presence of nitrogen,oxygen or an oxygen-containing gas. The calcination can be carried outin a muffle furnace, but better in a rotary kiln, in which thecorresponding gas flows continuously through the catalyst, which in turnresults in improved homogeneity of the catalyst compared with thecatalysts described in DE 19745902. The calcination duration is from 2to 24 hours.

[0033] The catalyst can be employed without a corresponding supportmaterial or mixed with one or applied to one. Conventional supportmaterials are suitable, such as, for example, porous silicon dioxide,ignited silicon dioxide, kieselguhr, silica gel, porous or nonporousaluminum oxide, titanium dioxide, zirconium dioxide, thorium dioxide,lanthanum oxide, magnesium oxide, calcium oxide, barium oxide, tinoxide, cerium dioxide, zinc oxide, boron oxide, boron nitride, boroncarbide, boron phosphate, zirconium phosphate, aluminum silicate,silicon nitride or silicon carbide, but also glass, carbon-fiber,metal-oxide or metal networks or corresponding monoliths.

[0034] Preferred support materials have a surface area of less than 100m²/g. Preferred support materials are silicon dioxide and aluminum oxideof low specific surface area. The catalyst can be employed after shapingas a regularly or irregularly shaped support element, but also in powderform as heterogeneous oxidation catalyst.

[0035] The reaction can be carried out in a fluidized bed or a fixed-bedreactor. For use in a fluidized bed, the catalyst is prepared byconventional processes, for example agglomeration, in such a way that apreferred particle size distribution in the range from 10 to 200 μm canbe achieved.

[0036] The gaseous feed comprises ethane and/or ethylene, which are fedto the reactor as pure gases or in the form of a mixture with one ormore other gases. Suitable as such additional or carrier gases are, forexample, nitrogen, methane, carbon monoxide, carbon dioxide, air and/orsteam. The gas containing molecular oxygen can be air or a gas which isricher or poorer in molecular oxygen than air, for example oxygen. Thecontent of steam can be in the range from 0 to 50% by volume. Highersteam concentrations would unnecessarily increase the cost of work-up ofthe aqueous acetic acid formed for technical reasons. For this reason,the steam concentration in the feed has been reduced further in theexamples of the present invention compared with the examples indicatedin DE 19745902, which would result in a significant cost saving in thework-up of acetic acid. The ratio between ethane/ethylene and oxygen isadvantageously in the range from 1:1 to 10:1, preferably from 2:1 to8:1. Higher oxygen contents are preferred, since the achievable ethaneconversion and thus the yield of acetic acid are higher. It is preferredto add oxygen or the gas containing molecular oxygen in a concentrationrange outside the explosion limits under reaction conditions, since thissimplifies performance of the process. However, it is also possible toset the ethane/ethylene to oxygen ratio within the explosion limits.

[0037] The reaction is carried out at temperatures of from 200 to 500°C., preferably from 200 to 400° C. The pressure may be atmospheric orsuperatmospheric, for example in the range from 1 to 50 bar, preferablyfrom 1 to 30 bar.

[0038] The reaction can be carried out in a fixed-bed or fluidized-bedreactor. In an advantageous procedure, ethane is firstly mixed with theinert gases, such as nitrogen or steam, before oxygen or the gascontaining molecular oxygen is fed in. The mixed gases are preferablypreheated to the reaction temperature in a preheating zone before thegas mixture is brought info contact with the catalyst. Acetic acid isseparated off from the reactor offgas by condensation. The remaininggases are fed back to the reactor inlets, where oxygen or the gascontaining molecular oxygen and ethane and/or ethylene is metered in.

[0039] In a comparison of the catalysts according to the invention withthose known from the prior art, it is found that higher space-timeyields and acetic-acid selectivities are achieved with the presentcatalysts under identical reaction conditions (reaction entry gas,pressure, temperature).

[0040] On use of the catalyst according to the invention, theselectivity in the oxidation of ethane and/or ethylene to acetic acid is≧70 mol %, preferably ≧80 mol %, in particular ≧90 mol %, and thespace-time yield is >470 kg/(hm³), in particular >500 kg/(hm³),preferably >550 kg/(hm³), enabling an increase in the acetic-acid yieldsto be achieved in a simple manner with simultaneous reduction in theamount of undesired by-products formed by means of the process accordingto the invention compared with the prior art.

EXAMPLES

[0041] The catalyst composition shown in the example is given inrelative atom ratios.

[0042] Catalyst Preparation:

[0043] Catalyst (I):

[0044] A catalyst of the following composition was prepared:

[0045] Mo_(1.000)Pd_(0.00075)V_(0.55)Nb_(0.09)Sb_(0.01)Ca_(0.01)O_(x)

[0046] Solution 1:

[0047] 80 g of ammonium molybdate (NH₄)₆Mo₇O₂₄×4H₂O (Riedel-de Haen) in400 ml of water.

[0048] Solution 2:

[0049] 29.4 g of ammonium metavanadate NH₄VO₃ (Riedel-de Haen) in 400 mlof water.

[0050] Solution 3:

[0051] 19.01 g of niobium ammonium oxalate (H. C. Starck),

[0052] 1.92 g of antimony oxalate Sb₂(C₂O₄)₃ (Pfaltz & Bauer),

[0053] 1.34 g of calcium nitrate Ca(NO₃)₂×4H₂O (Riedel-de Haen) in 200ml of water.

[0054] Solution 4:

[0055] 0.078 g of palladium(II) acetate (CH₃CO₂)₂Pd (Aldrich) in 200 mlof ethanol.

[0056] Aqueous solutions 1 to 3 are stirred separately at 70° C. for 15minutes. The third solution is then added to the second. The combinedmixtures are stirred at 70° C. for 15 minutes before these are added tothe first mixture. Solution 4 is then added. The resultant mixture isstirred at 70° C. for 15 minutes and subsequently evaporated to a volumeof 800 ml. The mixture is spray-dried and calcined in static air at 120°C. for 2 hours and at 300° C. for 5 hours. The catalyst is then groundslightly in a mortar and pressed to give tablets. These are crushed overa sieve in order to obtain a sieve fraction of from 0.35 to 0.7 mm.

[0057] Method for Catalyst Testing:

[0058] 5 or 10 ml of the catalyst were loaded into a steel reactor withan internal diameter of 14 mm. The catalyst was heated to 250° C. undera stream of air. The pressure was subsequently adjusted by means of aprepressure regulator. The desired ethane:oxygen:nitrogen mixture wasmetered into an evaporator zone with water, where water was evaporatedand mixed with the gases. The reaction temperature was measured in thecatalyst bed using a thermocouple. The reaction gas was analyzed on-lineby gas chromatography.

[0059] In the examples, the following terms are defined as:

Ethane Conversion(%)=([CO]/2+[CO₂]/2+[C₂H₄]+[CH₃COOH])/([CO]/₂+[CO₂]/2+[C₂H₄]+[C₂H₆]+[CH₃COOH])*100

Ethylene Selectivity (%)=([C₂H₄])/([CO]/₂+[CO₂]/2+[C₂H₄]+[CH₃COOH])*100

Acetic-Acid Selectivity(%)=([CH₃COOH])/([CO]/₂+[CO₂]/2+[C₂H₄]+[CH₃COOH])*100

[0060] in which

[0061] [ ]=concentration in mol %, and

[0062] [C₂H₆]=concentration of the unreacted ethane.

[0063] The residence time is defined as:

[0064] θ (s)=bulk volume of the catalyst (ml)/volume flow rate of thegas through the reactor based on the reaction conditions (ml/s),

[0065] STY denotes space-time yield in kg of acetic acid per hour and m³of catalyst.

[0066] Performance of the Reaction:

[0067] The ethane/oxygen/nitrogen ratio in the reaction gas was 5/1/4.The proportion of steam in the reaction mixture was set to values lessthan or equal to 20%. Since the space-time yield is dependent on thereaction pressure, all experimental examples were carried out at 15 barfor reasons of comparability. The reaction conditions and results areshown in Table 1. TABLE 1 Results of catalytic studies of the oxidationof ethane to acetic acid on catalyst (I) Reaction conditions ResultsSpace- Reaction gas composition Conversion Selectivity time yield Ex T θV(C₂H₆) V(O₂) V(N₂) V(H₂O) X(C₂H₆) S(HOAc) S(C₂H₄) S(CO + CO₂) STY(HOAc)No. [° C.] [s] [ml/s] [ml/s] [ml/s] [g/h] [%] [%] [%] [%] [kg/(hm³)] 1280 14.8 1.0 0.2 0.8 1.4 13.3 91.5 0.7 7.8 235 2 280 7.4 2.0 0.4 1.6 2.910.5 90.4 3.5 6.0 362 3 300 7.1 2.0 0.4 1.6 2.9 13.2 89.0 2.0 9.0 447 4300 4.8 3.0 0.6 2.4 4.3 11.3 87.2 5.5 7.3 564 5 300 4.1 3.5 0.7 2.8 5.010.2 86.2 7.4 6.4 584 6 300 3.7 4.0 0.8 3.2 5.0 9.9 84.1 9.2 6.6 630

[0068] Table 1 shows that the reduction in the residence time for thesame reaction gas composition and at a reaction temperature of 280° C.results in a negligible decrease in conversion, constant acetic-acidselectivity, but a one-and-a-half-fold increased space-time yield (cf.experiments 1 and 2). A further reduction in the residence time at 300°C. (cf. experiments 3 and 4) with comparable conversions andselectivities results in a further increase in the space-time yield.Compared with experiment 4, the water content in the starting gas streamis also reduced in experiments 5 and 6 in addition to a furtherreduction in the residence time. For comparable conversions andacetic-acid selectivities, it was possible to further increase thespace-time yield to 630 kg/(hm³). It should also be particularlyemphasized here that the marginal reduction in the acetic-acidselectivity compared with experiments 1 and 4 is not attributable toincreased formation of total oxidation products, but instead ethylene ispreferentially formed besides acetic acid owing to the low water contentin the feed and can be circulated as valuable product and furtheroxidized to acetic acid.

1. A continuous process for the selective preparation of acetic acidfrom a gaseous feed of ethane, ethylene or mixtures thereof and oxygenat elevated temperature, in which the gaseous feed is brought togetherwith a catalyst comprising the elements Mo, Pd, X and Y in gram-atomratios a:b:c:d in combination with oxygen Mo_(a)Pd_(b)X_(c)Y_(d)   (I)where the symbols X and Y are as defined below: X is one or moreelements selected from the group consisting of Cr, Mn, Nb, Ta, Ti, V, Teand W, but shall be at least Nb; Y is one or more elements selected fromthe group consisting of B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir,Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf, Ni, P,Pb, Sb, Si, Sn, Tl and U; the indices a, b, c, d and x are the gram-atomratios of the corresponding elements, where a=1; b=from 0.0001 to 0.01;c=from 0.4 to 1; and d=from 0.005 to 1, and in which the residence timesand the composition of the gaseous feed are selected in such a way thatthe space-time yield in the oxidation to acetic acid is >470 kg/(hm³),with the proviso that the catalyst is obtainable by the use of a niobiumammonium salt as niobium source.
 2. The process as claimed in claim 1,wherein X and/or Y are a plurality of elements, where, if desired, theindices c and d can adopt different values for different elements. 3.The process as claimed in at least one of claims 1 and 2, whereinniobium ammonium oxalate is used as niobium source.
 4. The process asclaimed in at least one of claims 1, 2 and 3, wherein the temperature isin the range from 200 to 500° C.
 5. The process as claimed in at leastone of claims 1 to 4, wherein the pressure in the reactor is in therange from 1 to 50 bar.
 6. The pressure as claimed in at least one ofclaims 1 to 5, wherein b is in the range from 0.0001 to 0.001.
 7. Theprocess as claimed in at least one of claims 1 to 6, wherein ethanemixed with at least one further gas is fed to the reactor.
 8. Theprocess as claimed in claim 7, wherein the further gas fed in isnitrogen, oxygen, methane, carbon monoxide, carbon dioxide, ethyleneand/or steam.
 9. The process as claimed in at least one of claims 1 to8, wherein the catalyst is mixed with a support material or immobilizedon a support material.
 10. The process as claimed in at least one ofclaims 1 to 9, wherein the selectivity of the oxidation reaction ofethane and/or ethylene to acetic acid is >70 mol %.
 11. The process asclaimed in at least one of claims 1 to 10, wherein the catalystcomprises Pd introduced in the form of an alcoholic solution ofpalladium acetate.
 12. A catalyst for tlhe selective oxidation ofethane, ethylene or mixtures thereof and oxygen, comprising the elementsMo, Pd, X and Y in gram-atom ratios a:b:c:d in combination with oxygenMo_(a)Pd_(b)X_(c)Y_(d)   (I) where the symbols X and Y are as definedbelow: X is one or more elements selected from the group consisting ofCr, Mn, Nb, Ta, Ti, V and W, shall be at least Nb; Y is one or moreelements selected from the group consisting of B, Al, Ga, In, Pt, Zn,Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr,Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl and U; the indices a, b, c and dare the gram-atom ratios of the corresponding elements, where a=1;b=from 0.0001 to 0.01; c=from 0.4 to 1; and d=from 0.005 to 1, and thespace-time yield in the oxidation reaction is >470 kg/(hm³), with theproviso that the catalyst is obtainable by the use of a niobium ammoniumsalt as niobium source.
 13. A catalyst as claimed in claim 12, whereniobium ammonium oxalate is used as niobium source.
 14. The catalyst asclaimed in claim 12, where the Pd has been introduced in the form of analcoholic solution of palladium acetate.